In x-ray diagnostics, contrasts in image data are produced by different attenuation characteristics of the substances with respect to the x-ray radiation generated by an x-ray device. Organs, vessels or tumors have a similar attenuation characteristic compared to neighboring tissue structures and, as a result of the low contrast connected to this, cannot be examined precisely in the image data without undertaking further measures. It is for this reason that a contrast agent having a very different attenuation characteristic compared to the tissue is utilized in these cases in order to increase the contrast. Hence, tissue structures can indirectly be made visible via the distribution of the contrast agent in the acquired image.
For this purpose, a contrast agent is administered to the patient according to a predeterminable injection protocol before the examination commences, with the contrast agent spreading through the examination region via the cardiovascular system. The spreading of the contrast agent in the body of the patient is a highly dynamic process. The amount of contrast agent present in the examination region and the increased contrast in the image data produced by the contrast agent depend on a multiplicity of patient-specific and examination-specific parameters. By way of example, examination-specific influencing variables include the concentration of the substance by which the attenuation characteristic of the contrast agent is determined, the absolute amount of injected volume or the flow rate of the contrast agent during the injection itself. Moreover, the pervasion of contrast agent into the examination region also depends on the selected period of time between the injection and the scan for obtaining image data and on the position on the body of the patient at which the contrast agent is administered. By way of example, patient-specific parameters include the heart rate and/or the blood pressure of the patient at the time of the examination. Moreover, the dispersion behavior of the contrast agent can also change as a result of taking medication.
Tumor diagnostics has a particular significance in human medicine because tumor-like diseases represent the second highest cause of death in industrialized countries after diseases of the cardiovascular system. The crucial basis for fighting such diseases is exact diagnosis with regard to tumor grading (diagnosis of type) and tumor staging (spread). In tumor staging, image data is compared to other image data recorded in examinations at different times in order to analyze the spread of the diseased tissue. The period between subsequent examinations can be weeks, months or even years.
As a result of the previously described influencing variables in imaging using an x-ray tomography scanner and a contrast agent, it is generally difficult to provide analysis of the tumor spread velocity on the basis of a comparison of image data because subsequent examinations cannot be carried out under exactly the same examination conditions. Often, it is not possible for the medical practitioner undertaking the diagnosis to discern whether the observed changes in contrast in the image data can be traced back to cell growth or are due to the follow-up examination having changed patient-specific and/or examination-specific parameters.